Custom errors should inherit from `StandardError` (not `Exception`)

[generalmimon]

See https://www.honeybadger.io/blog/ruby-exception-vs-standarderror-whats-the-difference/#custom-exceptions-should-inherit-from-standarderror:

It means you should always inherit from StandardError, and NEVER from Exception. Inheriting from Exception is bad because it breaks the expected behavior of rescue. People will think they're rescuing all application-level errors but yours will just sail on through.

https://github.com/kaitai-io/kaitai_struct_ruby_runtime/blob/ecf67519a990296bb1e095363ef2dc414e600502/lib/kaitai/struct/struct.rb#L688-L692

But UnexpectedDataError should be left as is, because it only remains for compatibility with 0.8 and older KS versions (and will be eventually removed):

https://github.com/kaitai-io/kaitai_struct_ruby_runtime/blob/ecf67519a990296bb1e095363ef2dc414e600502/lib/kaitai/struct/struct.rb#L78-L84

[generalmimon]

Yeah, this issue is also reported by RuboCop:

lib/kaitai/struct/struct.rb:692:27: W: [Correctable] Lint/InheritException: Inherit from StandardError instead of Exception.
class KaitaiStructError < Exception
                          ^^^^^^^^^

We should adopt https://github.com/kaitai-io/kaitai_struct_visualizer/issues/38 for runtime library too, and make sure that RuboCop at least doesn't issue any warnings.

[GreyCat]

Thanks for tracking and fixing this!